Check number of arguments passed to a Bash script

Question

I would like my Bash script to print an error message if the required argument count is not met   I tried the following code      bin bash echo Script name   0 echo    arguments  if     -ne 1        then echo  illegal number of parameters  fi   For some unknown reason I ve got the following error   test  line 4   2  command not found   What am I doing wrong

User · Answer

It might be a good idea to use arithmetic expressions if you're dealing with numbers.

if (( $# != 1 )); then
    >&2 echo "Illegal number of parameters"
fi

>&2 is used to write the error message to stderr.

User · Answer

A simple one liner that works can be done using     quot    quot  -ne 1    amp  amp    usage  amp  amp  exit 1      main  This breaks down to   test the bash variable for size of parameters    not equals 1  our number of sub commands  if true then call usage   function and exit with status 1 else call main   function  Things to note   usage   can  just be simple echo  quot  0  params quot  main can be one long script

User · Answer

Just like any other simple command          or test requires spaces between its arguments  if    quot    quot  -ne 1    then     echo  quot Illegal number of parameters quot  fi  Or if test  quot    quot  -ne 1  then     echo  quot Illegal number of parameters quot  fi  Suggestions When in Bash  prefer using       instead as it doesn t do word splitting and pathname expansion to its variables that quoting may not be necessary unless it s part of an expression        -ne 1     It also has some other features like unquoted condition grouping  pattern matching  extended pattern matching with extglob  and regex matching  The following example checks if arguments are valid  It allows a single argument or two         -eq 1        -eq 2  amp  amp   2     lt glob pattern gt     amp  amp   1     lt regex pattern gt      For pure arithmetic expressions  using       to some may still be better  but they are still possible in       with its arithmetic operators like -eq  -ne  -lt  -le  -gt  or -ge by placing the expression as a single string argument  A 1     A   1  -eq 2     amp  amp  echo true     Prints true   That should be helpful if you would need to combine it with other features of       as well  Take note that       and       are keywords which have same level of parsing as if  case  while  and for  Also as Dave suggested  error messages are better sent to stderr so they don t get included when stdout is redirected  echo  quot Illegal number of parameters quot   gt  amp 2  Exiting the script It s also logical to make the script exit when invalid parameters are passed to it   This has already been suggested in the comments by ekangas but someone edited this answer to have it with -1 as the returned value  so I might as well do it right  -1 though accepted by Bash as an argument to exit is not explicitly documented and is not right to be used as a common suggestion   64 is also the most formal value since it s defined in sysexits h with  define EX USAGE 64    command line usage error      Most tools like ls also return 2 on invalid arguments   I also used to return 2 in my scripts but lately I no longer really cared  and simply used 1 in all errors   But let s just place 2 here since it s most common and probably not OS-specific  if       -ne 1     then     echo  quot Illegal number of parameters quot      exit 2 fi  References  Bash Conditional Expressions Conditional Constructs Pattern Matching Word Splitting Filename Expansion  prev  Pathname Expansion  Simple Commands

User · Answer

If you re only interested in bailing if a particular argument is missing  Parameter Substitution is great      bin bash   usage-message sh      1  Usage   0 ARGUMENT      Script exits here if command-line parameter absent     with following error message       usage-message sh  1  Usage  usage-message sh ARGUMENT

User · Answer

You should add spaces between test condition   if      -ne 1         then echo  illegal number of parameters  fi   I hope this helps

User · Answer

There is a lot of good information here  but I wanted to add a simple snippet that I find useful  How does it differ from some above   Prints usage to stderr  which is more proper than printing to stdout Return with exit code mentioned in this other answer Does not make it into a one liner      usage         echoerr  quot Usage   0  lt args gt  quot      echoerr        echo  quot    quot   gt  amp 2    if    quot    quot  -eq 0    then   NOTE  May need to customize this conditional      usage     exit 2 fi main  quot    quot

User · Answer

In case you want to be on the safe side  I recommend to use getopts    Here is a small example        while getopts  x c  opt  do       case  opt in         c            echo  - opt was triggered  deploy to ci account   gt  amp 2           DEPLOY CI ACCT  true                           x                echo  - opt was triggered  Parameter   OPTARG   gt  amp 2                CMD TO EXEC   OPTARG                                                 echo  Invalid option  - OPTARG   gt  amp 2                Usage               exit 1                                               echo  Option - OPTARG requires an argument    gt  amp 2                Usage               exit 1                            esac         done   see more details here for example http   wiki bash-hackers org howto getopts tutorial

User · Answer

Check out this bash cheatsheet  it can help alot   To check the length of arguments passed in  you use       To use the array of arguments passed in  you use       An example of checking the length  and iterating would be   myFunc       if         -gt 0     then     for arg in       do       echo  arg     done   fi    myFunc        This articled helped me  but was missing a few things for me and my situation  Hopefully this helps someone

User · Answer

Here a simple one liners to check if only one parameter is given otherwise exit the script          -ne 1    amp  amp  echo  USAGE  0  lt PARAMETER gt    amp  amp  exit

User · Answer

On                   are string comparison operators and -eq  -gt     are arithmetic binary ones   I would use   if            1     then   Or   if      -eq 1    then

[bash] Check number of arguments passed to a Bash script

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