Meaning of “! -S” in shell script

The command you are looking at is actually this:

[ ! -S "qdata/c$i/tm.ipc" ]

Although it looks like punctuation, [ is actually the name of a command, also called test; so the command can also be written like this:

test ! -S "qdata/c$i/tm.ipc"

Which in context would look like this:

if test ! -S "qdata/c$i/tm.ipc"; then
        DOWN=true
fi

As the name suggests, its job is to test some attribute of a string, number, or file, and return 0 (which represents true in shell scripts) if the test passes, and 1 (which represents false) if it doesn’t.

Armed with this knowledge, you can run man test, and find the following explanations of the ! and -S arguments:

   ! EXPRESSION
         EXPRESSION is false

and

   -S FILE
         FILE exists and is a socket

So test ! -S filename or [ ! -S filename ] can be read as “not is-socket filename”.

So the command is checking whether a “socket” (a special kind of file) exists with each name in the loop. The script uses this command as the argument to an if statement (which can take any command, not just [) and sets DOWN to true if any of them does not exist.

Related Posts:

  1. “No such file or directory” but it exists
  2. How do I split a string on a delimiter in Bash?
  3. Ubuntu says “bash: ./program Permission denied” [closed]
  4. Should I put #! (shebang) in Python scripts, and what form should it take?
  5. Should I put #! (shebang) in Python scripts, and what form should it take?
  6. How do I copy a folder from remote to local using scp?
  7. How to concatenate string variables in Bash
  8. -bash: syntax error near unexpected token `)’
  9. An example of how to use getopts in bash
  10. How to delete from a text file, all lines that contain a specific string?
  11. How to mkdir only if a directory does not already exist?
  12. How do I iterate over a range of numbers defined by variables in Bash?
  13. How to set ssh timeout?
  14. commands not found on zsh
  15. sudo: apt-get: command not found
  16. How do I iterate over a range of numbers defined by variables in Bash?
  17. Why do people write #!/usr/bin/env python on the first line of a Python script?
  18. How to mkdir only if a directory does not already exist?
  19. Using find to locate files that match one of multiple patterns
  20. “unary operator expected” error in Bash if condition
  21. What does “-ne” mean in bash?
  22. Is there a Python equivalent to the ‘which’ command
  23. How to generate random number in Bash?
  24. How to check if a variable is set in Bash?
  25. Who can access a file with octal permissions “000” on Linux/UNIX?
  26. How can I compare numbers in Bash?
  27. What does the line “#!/bin/sh” mean in a UNIX shell script?
  28. Open and write data to text file using Bash?
  29. How do I set a variable to the output of a command in Bash?
  30. python getoutput() equivalent in subprocess
  31. What is the $? (dollar question mark) variable in shell scripting?
  32. Is there a TRY CATCH command in Bash
  33. dquote> result of a execution a program in linux shell
  34. Using sudo with Python script
  35. how to fix the issue “Command /bin/sh failed with exit code 1” in iphone
  36. Run a string as a command within a Bash script
  37. conditional binary operator expected in shell script
  38. Block Comments in a Shell Script
  39. How to get a shell environment variable in a makefile?
  40. What are the uses of the exec command in shell scripts? 
  41. using rot13 and tr command for having an encrypted email address
  42. Shell Script: Execute a python program from within a shell script
  43. sed whole word search and replace
  44. What’s the difference betwen the single dash and double dash flags on shell commands?
  45. How to let ‘cp’ command don’t fire an error when source file does not exist?
  46. What is a reverse shell?
  47. How do I pause my shell script for a second before continuing?
  48. How to execute a program or call a system command?
  49. In the shell, what does ” 2>&1 ” mean?
  50. Multi-line string with extra space (preserved indentation)
  51. How to perform grep operation on all files in a directory?
  52. ‘\r’: command not found – .bashrc / .bash_profile [duplicate]
  53. OS X: equivalent of Linux’s wget
  54. subprocess.Popen(): OSError: [Errno 8] Exec format error in python?
  55. How to substitute shell variables in complex text files
  56. How can I recursively find all files in current and subfolders based on wildcard matching?
  57. Curl command for https ( SSL )
  58. How to exclude a directory in find . command
  59. “sed” command in bash
  60. How can I declare and use Boolean variables in a shell script?
  61. How to exclude a directory in find . command
  62. How to reload .bash_profile from the command line?
  63. What is the purpose of “&&” in a shell command?
  64. Shell Script Syntax Error: Unexpected End of File
  65. “[ ]” vs. “[[ ]]” in Bash shell
  66. How do I split a string on a delimiter in Bash?
  67. adb shell su works but adb root does not
  68. What is a list in Bash?
  69. Elegant way to search for UTF-8 files with BOM?
  70. Given two directory trees, how can I find out which files differ by content?
  71. adb shell su works but adb root does not
  72. How to remove the quotes when reading a variable in jq in shell?
  73. Shell script “for” loop syntax
  74. Pseudo-terminal will not be allocated because stdin is not a terminal
  75. screen Cannot open your terminal ‘/dev/pts/0’ – please check
  76. ./configure : /bin/sh^M : bad interpreter
  77. What is the difference between “#!/usr/bin/env bash” and “#!/usr/bin/bash”?
  78. How to convert a string to lower case in Bash?
  79. Is there a “goto” statement in bash?
  80. Command to change the default home directory of a user
  81. Sorting data based on second column of a file
  82. zip error – Nothing to do
  83. What does “&” at the end of a linux command mean?
  84. How to sort an array in Bash
  85. Run script on mac prompt “Permission denied”
  86. * failed to open vchiq instance
  87. how to check which version of nltk, scikit learn installed?
  88. How to split one string into multiple strings separated by at least one space in bash shell?
  89. Copy multiple files from one directory to another from Linux shell
  90. execvp: bad address error
  91. Using grep and ls -a commands
  92. How to use sed to extract substring
  93. Running shell command and capturing the output
  94. Simple PowerShell LastWriteTime compare
  95. Can pm2 run an ‘npm start’ script
  96. Integer expression expected error in shell script
  97. Shell command to monitor changes in a file
  98. How to run command as user who has /usr/sbin/nologin as Shell?
  99. Adding a directory to $PATH in CentOS?
  100. How do I join two named pipes into single input stream in linux

Leave a Comment