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9

Problem

I tried to extract the signal level for wifi in the following way, in a bash script (quotes seem to make no difference):

string="$(iwconfig wlan0 | grep -I Signal)"

If I echo $string, I get what I expect:

Link Quality=63/70 Signal level=-47dBm

But if I try to get a substring, echo ${string:5}, it returns the same thing.

---

Debuging:

If I paste the output of wconfig wlan0 | grep -I Signal directly into the variable: string="Link Quality=63/70 Signal level=-47dBm" then everything works as expected.

echo $string 

# Link Quality=63/70 Signal level=-47dBm

echo ${string:5}

# Quality=63/70 Signal level=-47dBm

---

Question: According to the internet, all bash variables are stored as character strings. Thus, the debugging result should have been the same as my original problem. Is there some reason it is not parsed as text in the original problem?

command-line bash scripts text

share|improve this question

edited 11 hours ago

user3140225

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Ion SmeIon Sme

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9

Problem

I tried to extract the signal level for wifi in the following way, in a bash script (quotes seem to make no difference):

string="$(iwconfig wlan0 | grep -I Signal)"

If I echo $string, I get what I expect:

Link Quality=63/70 Signal level=-47dBm

But if I try to get a substring, echo ${string:5}, it returns the same thing.

---

Debuging:

If I paste the output of wconfig wlan0 | grep -I Signal directly into the variable: string="Link Quality=63/70 Signal level=-47dBm" then everything works as expected.

echo $string 

# Link Quality=63/70 Signal level=-47dBm

echo ${string:5}

# Quality=63/70 Signal level=-47dBm

---

Question: According to the internet, all bash variables are stored as character strings. Thus, the debugging result should have been the same as my original problem. Is there some reason it is not parsed as text in the original problem?

command-line bash scripts text

share|improve this question

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

New contributor

Ion Sme is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

Problem

I tried to extract the signal level for wifi in the following way, in a bash script (quotes seem to make no difference):

string="$(iwconfig wlan0 | grep -I Signal)"

If I echo $string, I get what I expect:

Link Quality=63/70 Signal level=-47dBm

But if I try to get a substring, echo ${string:5}, it returns the same thing.

---

Debuging:

If I paste the output of wconfig wlan0 | grep -I Signal directly into the variable: string="Link Quality=63/70 Signal level=-47dBm" then everything works as expected.

echo $string 

# Link Quality=63/70 Signal level=-47dBm

echo ${string:5}

# Quality=63/70 Signal level=-47dBm

---

Question: According to the internet, all bash variables are stored as character strings. Thus, the debugging result should have been the same as my original problem. Is there some reason it is not parsed as text in the original problem?

command-line bash scripts text

share|improve this question

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

New contributor

Ion Sme is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

string="$(iwconfig wlan0 | grep -I Signal)"

Link Quality=63/70 Signal level=-47dBm

echo $string 

# Link Quality=63/70 Signal level=-47dBm

echo ${string:5}

# Quality=63/70 Signal level=-47dBm

share|improve this question

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

New contributor

Ion Sme is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

New contributor

Ion Sme is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

edited 11 hours ago

user3140225

66333 silver badges1717 bronze badges

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

New contributor

Ion Sme is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 12 hours ago

Ion SmeIon Sme

4844 bronze badges

2 Answers
2

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9

This is because echo $string doesn't simply print the value of string; it first perform a split+glob operation - one of the consequences of which is that leading whitespace is elided:

$ string=$(iwconfig wls1 | grep Signal)

$ echo $string

Link Quality=38/70 Signal level=-72 dBm

whereas

$ echo "$string"

          Link Quality=38/70  Signal level=-72 dBm  

We can see that there is a sequence of whitespace characters at the start of $string - in fact there are more than 5, so removing 5 still leaves a string with leading whitespace, which an unquoted echo also elides:

$ echo "${string:5}"

     Link Quality=38/70  Signal level=-72 dBm  

$ echo ${string:5}

Link Quality=38/70 Signal level=-72 dBm

For additional discussion see:

• When is double-quoting necessary?




• Why does my shell script choke on whitespace or other special characters?

share|improve this answer

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Tl;dr always quote your variables
  
  – gronostaj
  55 mins ago
  

  
  
  
  

add a comment | 

3

This works on my machine:

$ string="$(iwconfig wlp60s0 | grep -I Signal)"

$ echo $string

Link Quality=68/70 Signal level=-42 dBm

$ echo $string | cut -d' ' -f4,5

level=-42 dBm

• For your machine replace wlp60s0 with wlan0.


• 
  Note this is using Ubuntu 16.04 and Ubuntu 19.04 where there is a space between 42 and dBm. As such the cut command is instructed to print fields #4 and #5. In your question there is no space so I'm not sure what version you are using.

You could also use:

$ echo $string | cut -d'=' -f3

-42 dBm

• In this case cut is told to use = as field delimiter.

If you want to use ${string...} though the correct syntax is:

$ echo ${string##*=}

-38 dBm



$ echo "${string##*=}"

-38 dBm  

Either method will work to take the substring after the last =. The original method of 5 in your question I don't understand how it can work.

share|improve this answer

edited 10 hours ago

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

add a comment | 

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9

This is because echo $string doesn't simply print the value of string; it first perform a split+glob operation - one of the consequences of which is that leading whitespace is elided:

$ string=$(iwconfig wls1 | grep Signal)

$ echo $string

Link Quality=38/70 Signal level=-72 dBm

whereas

$ echo "$string"

          Link Quality=38/70  Signal level=-72 dBm  

We can see that there is a sequence of whitespace characters at the start of $string - in fact there are more than 5, so removing 5 still leaves a string with leading whitespace, which an unquoted echo also elides:

$ echo "${string:5}"

     Link Quality=38/70  Signal level=-72 dBm  

$ echo ${string:5}

Link Quality=38/70 Signal level=-72 dBm

For additional discussion see:

• When is double-quoting necessary?




• Why does my shell script choke on whitespace or other special characters?

share|improve this answer

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Tl;dr always quote your variables
  
  – gronostaj
  55 mins ago
  

  
  
  
  

add a comment | 

9

This is because echo $string doesn't simply print the value of string; it first perform a split+glob operation - one of the consequences of which is that leading whitespace is elided:

$ string=$(iwconfig wls1 | grep Signal)

$ echo $string

Link Quality=38/70 Signal level=-72 dBm

whereas

$ echo "$string"

          Link Quality=38/70  Signal level=-72 dBm  

We can see that there is a sequence of whitespace characters at the start of $string - in fact there are more than 5, so removing 5 still leaves a string with leading whitespace, which an unquoted echo also elides:

$ echo "${string:5}"

     Link Quality=38/70  Signal level=-72 dBm  

$ echo ${string:5}

Link Quality=38/70 Signal level=-72 dBm

For additional discussion see:

• When is double-quoting necessary?




• Why does my shell script choke on whitespace or other special characters?

share|improve this answer

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Tl;dr always quote your variables
  
  – gronostaj
  55 mins ago
  

  
  
  
  

add a comment | 

This is because echo $string doesn't simply print the value of string; it first perform a split+glob operation - one of the consequences of which is that leading whitespace is elided:

$ string=$(iwconfig wls1 | grep Signal)

$ echo $string

Link Quality=38/70 Signal level=-72 dBm

whereas

$ echo "$string"

          Link Quality=38/70  Signal level=-72 dBm  

We can see that there is a sequence of whitespace characters at the start of $string - in fact there are more than 5, so removing 5 still leaves a string with leading whitespace, which an unquoted echo also elides:

$ echo "${string:5}"

     Link Quality=38/70  Signal level=-72 dBm  

$ echo ${string:5}

Link Quality=38/70 Signal level=-72 dBm

For additional discussion see:

• When is double-quoting necessary?




• Why does my shell script choke on whitespace or other special characters?

share|improve this answer

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

$ string=$(iwconfig wls1 | grep Signal)

$ echo $string

Link Quality=38/70 Signal level=-72 dBm

$ echo "$string"

          Link Quality=38/70  Signal level=-72 dBm  

$ echo "${string:5}"

     Link Quality=38/70  Signal level=-72 dBm  

$ echo ${string:5}

Link Quality=38/70 Signal level=-72 dBm

share|improve this answer

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

answered 11 hours ago

steeldriversteeldriver

77.2k1212 gold badges128128 silver badges206206 bronze badges

3

This works on my machine:

$ string="$(iwconfig wlp60s0 | grep -I Signal)"

$ echo $string

Link Quality=68/70 Signal level=-42 dBm

$ echo $string | cut -d' ' -f4,5

level=-42 dBm

• For your machine replace wlp60s0 with wlan0.


• 
  Note this is using Ubuntu 16.04 and Ubuntu 19.04 where there is a space between 42 and dBm. As such the cut command is instructed to print fields #4 and #5. In your question there is no space so I'm not sure what version you are using.

You could also use:

$ echo $string | cut -d'=' -f3

-42 dBm

• In this case cut is told to use = as field delimiter.

If you want to use ${string...} though the correct syntax is:

$ echo ${string##*=}

-38 dBm



$ echo "${string##*=}"

-38 dBm  

Either method will work to take the substring after the last =. The original method of 5 in your question I don't understand how it can work.

share|improve this answer

edited 10 hours ago

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

add a comment | 

3

This works on my machine:

$ string="$(iwconfig wlp60s0 | grep -I Signal)"

$ echo $string

Link Quality=68/70 Signal level=-42 dBm

$ echo $string | cut -d' ' -f4,5

level=-42 dBm

• For your machine replace wlp60s0 with wlan0.


• 
  Note this is using Ubuntu 16.04 and Ubuntu 19.04 where there is a space between 42 and dBm. As such the cut command is instructed to print fields #4 and #5. In your question there is no space so I'm not sure what version you are using.

You could also use:

$ echo $string | cut -d'=' -f3

-42 dBm

• In this case cut is told to use = as field delimiter.

If you want to use ${string...} though the correct syntax is:

$ echo ${string##*=}

-38 dBm



$ echo "${string##*=}"

-38 dBm  

Either method will work to take the substring after the last =. The original method of 5 in your question I don't understand how it can work.

share|improve this answer

edited 10 hours ago

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

add a comment | 

This works on my machine:

$ string="$(iwconfig wlp60s0 | grep -I Signal)"

$ echo $string

Link Quality=68/70 Signal level=-42 dBm

$ echo $string | cut -d' ' -f4,5

level=-42 dBm

• For your machine replace wlp60s0 with wlan0.


• 
  Note this is using Ubuntu 16.04 and Ubuntu 19.04 where there is a space between 42 and dBm. As such the cut command is instructed to print fields #4 and #5. In your question there is no space so I'm not sure what version you are using.

You could also use:

$ echo $string | cut -d'=' -f3

-42 dBm

• In this case cut is told to use = as field delimiter.

If you want to use ${string...} though the correct syntax is:

$ echo ${string##*=}

-38 dBm



$ echo "${string##*=}"

-38 dBm  

Either method will work to take the substring after the last =. The original method of 5 in your question I don't understand how it can work.

share|improve this answer

edited 10 hours ago

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

$ string="$(iwconfig wlp60s0 | grep -I Signal)"

$ echo $string

Link Quality=68/70 Signal level=-42 dBm

$ echo $string | cut -d' ' -f4,5

level=-42 dBm

$ echo $string | cut -d'=' -f3

-42 dBm

$ echo ${string##*=}

-38 dBm



$ echo "${string##*=}"

-38 dBm  

share|improve this answer

edited 10 hours ago

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges

answered 11 hours ago

WinEunuuchs2UnixWinEunuuchs2Unix

56.1k1616 gold badges109109 silver badges217217 bronze badges