Script di shell: opzioni -z e -n con if

Puoi trovare un bel riferimento per gli operatori di bash qui . Se utilizzi una shell diversa, cerca <my shell> operators e troverai tutto ciò di cui hai bisogno. Nel tuo caso particolare, sei utilizzando:

-n string is not null. -z string is null, that is, has zero length 

Per illustrare:

$ foo="bar"; $ [ -n "$foo" ] && echo "foo is not null" foo is not null $ [ -z "$foo" ] && echo "foo is null" $ foo=""; $ [ -n "$foo" ] && echo "foo is not null" $ [ -z "$foo" ] && echo "foo is null" foo is null 

Commenti

• Se hai problemi a far funzionare -n, potrebbe essere perché stai seguendo alcune cattive guide sul Web (ad esempio GeeksforGeeks o TutorialsPoint ) che non citano le variabili. Questa risposta e la guida collegata qui citano correttamente Se utilizzi -n senza virgolette, ti dirà che ' non è vuoto anche quando lo è! @terdon, grazie mille!

Per estendere la risposta di terdon “, ho scoperto che Unix / Linux – Shell Basic Operators on Tutorials Point include anche operatori relativi ai file (oltre ad altri utili).

-b file Checks if file is a block special file; if yes, then the condition becomes true. [ -b $file ] is false. -c file Checks if file is a character special file; if yes, then the condition becomes true. [ -c $file ] is false. -d file Checks if file is a directory; if yes, then the condition becomes true. [ -d $file ] is not true. -f file Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true. [ -f $file ] is true. -g file Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true. [ -g $file ] is false. -k file Checks if file has its sticky bit set; if yes, then the condition becomes true. [ -k $file ] is false. -p file Checks if file is a named pipe; if yes, then the condition becomes true. [ -p $file ] is false. -t file Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true. [ -t $file ] is false. -u file Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true. [ -u $file ] is false. -r file Checks if file is readable; if yes, then the condition becomes true. [ -r $file ] is true. -w file Checks if file is writable; if yes, then the condition becomes true. [ -w $file ] is true. -x file Checks if file is executable; if yes, then the condition becomes true. [ -x $file ] is true. -s file Checks if file has size greater than 0; if yes, then condition becomes true. [ -s $file ] is true. -e file Checks if file exists; is true even if file is a directory but exists. [ -e $file ] is true.